Influence of Surface Crystalline Structures on DSC ... Influence of Surface Crystalline Structures

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  • DOI: http://dx.doi.org/10.1590/1980-5373-MR-2017-0326Materials Research. 2017; 20(5): 1350-1359 2017

    Influence of Surface Crystalline Structures on DSC Analysis of PTFE

    Vinicius Fiocco Sciutia*, Caiu Caldeira Meloa, Leonardo Bresciani Cantoa, Rodrigo Bresciani Cantoa

    Received: March 28, 2017; Revised: June 21, 2017; Accepted: July 03, 2017

    The physical and mechanical properties of polytetrafluorethylene (PTFE) are greatly dependent on the degree of crystallinity and this is extremely important for the modeling of PTFE processing which is complex and costly. Differential scanning calorimetry (DSC) is one of the most important techniques for the determination of the degree of crystallinity and powder granules of the sample are generally used in the analysis. This procedure provides samples with a high surface-to-volume ratio, resulting in the formation of a considerable number of surface crystalline structures, called warts, along with the bulk crystallization, as shown by scanning electron microscopy. The presence of warts has a significant effect on the PTFE melting enthalpy and thus hinders the correct estimation of the degree of crystallinity of industrial PTFE parts, in which bulk crystallization prevails. In this study, we propose a procedure which does not lead to the formation of warts in the DSC sample and thus allows a more accurate determination of the melting enthalpy (or the degree of crystallinity) of industrial PTFE parts. We demonstrate that samples must be extracted from the core of dense (well-pressed) parts previously sintered in an oven, and the use of powder granules and/or sintering in DSC is not recommended.

    Keywords: PTFE, DSC, degree of crystallinity, surface crystallization, warts.

    * e-mail: vinicius@ppgcem.ufscar.br

    1. Introduction

    Components and parts made of polytetrafluorethylene (PTFE) are expensive due to the high cost associated with this polymer and its processing. Because of its high melt viscosity (109 to 1011 Pa s-1 at 360 oC1), PTFE parts are not manufactured via the traditional melt processing routes for thermoplastic polymers. Instead, PTFE powder granules are cold pressed followed by hot sintering1-5. Sintering is usually carried out at temperatures up to 375 oC (i.e., above the PTFE melting temperature) for between 10 and 10,000 min depending on the size of the specimen, followed by cooling at slow rates, typically in the range of 0.1 to 0.8 oC min-1, to avoid the formation of microcracks in the specimen. As-received PTFE has a high melting temperature (Tm) of around 342 oC and high degree of crystallinity, in the range of 89 to 98%2,3. After sintering, Tm is reduced to 327

    oC and the degree of crystallinity decreases to values between 38 and 53% depending on the conditions6. This is attributed to the high degree of conformational constraint of the PTFE chain during cooling from the melt, which hinders the crystallization7.

    The outstanding properties of this polymer allow PTFE parts to be used under critical service conditions. In the automotive industry, bearings and bushings made of PTFE are used without additional lubrication because of the good mechanical properties of the polymer along with its low friction coefficient when in contact with a range of surfaces8. The high electrical resistivity and thermal

    stability of PTFE make it suitable for use in electrical wire and cable insulations2. In the chemical industry, gaskets, vessels, valves and pipes are made of PTFE because of its chemical inertness and high thermal stability, among other properties. All of these PTFE properties are significantly affected by the degree of crystallinity after sintering6,9,10. The performance of PTFE specimens in mechanical testing is also influenced by the degree of crystallinity6,9-12. Rae and Dattelbaum9, for instance, observed an increase of 18% in the yield stress of PTFE in compression tests with an increase in the degree of crystallinity from 32 to 48%. The tensile behavior of PTFE has also been found to be affected by the degree of crystallinity6. Samples with a low degree of crystallinity present higher stiffness and larger deformations than those with a high degree of crystallinity. In the early stages of tensile loading, the PTFE chains in the amorphous phase align in the tensile loading direction. After reaching a certain amount of deformation, the PTFE crystals can slide, stretch and orientate in the direction of the tensile axis6. With regard to the fracture behavior, PTFE undergoes localized plastic deformation in the vicinity of microvoids above 19 oC, leading to the formation of fibrils, which results in the dissipation of energy11. An increase in the degree of crystallinity of PTFE reduces the fibrillation, thus decreasing the fracture toughness of the material13. In this regard, the crack surface displacement mode plays an important role, that is, the formation of fibrils is more pronounced in mode-I than mode-II (shear)14. The creep behavior of PTFE are also affected by the degree of crystallinity15, along with the

    aDepartamento de Engenharia de Materiais, Programa de Ps-Graduao em Cincia e Engenharia de Materiais - PPGCEM, Universidade Federal de So Carlos - UFSCar, So Carlos, SP, Brasil

  • 1351Influence of Surface Crystalline Structures on DSC Analysis of PTFE

    coefficient of thermal expansion3. All of the above-mentioned properties are relevant for the design of PTFE components and parts, which emphasizes the importance of the accurate estimation of the crystallinity degree of PTFE as a function of the heat cycle imposed during sintering. The fact that PTFE processing is complex and costly, along with the significant dependence of the mechanical properties on the degree of crystallinity, has motivated researchers to carry out studies on the characterization and computational modeling of cold pressing16,17 and sintering3,18,19.

    In several studies reported in the literature differential scanning calorimetry (DSC) was used to investigate the melting3-5,7,16,20-23 and crystallization3-5,15,16,19-26 of PTFE.

    Lau and co-workers24 used DSC to measure the heat capacity and phase-change enthalpies of two sets of PTFE samples with different degrees of crystallinity in the temperature interval of -103 oC to 427 oC. The authors do not mention any processing involving pressing or sintering before the analysis, suggesting that as-received powder granules were used. The as-received samples were considered to be 100% crystalline and a less crystalline sample was obtained through heat treatment at 352oC for 15 min followed by cooling at 43 oC min-1 prior to the analysis. The authors suggested that the data reported could be used to update and expand the information on the thermodynamic properties of PTFE available in the literature.

    Mantuano and Gomes5 studied the effects of cold pressing and sintering time on the microstructure of PTFE parts. They obtained cylinder-shaped specimens by isostatic pressing and disc-shaped samples were extracted from these cylinders. To simulate the sintering, the discs were heat-treated by DSC at temperatures between 365 and 380 oC for time intervals of 5 to 60 min. Thereafter, the crystallization enthalpy was measured by DSC to estimate the dependence of the degree of crystallinity on these sintering conditions. The degree of crystallinity of PTFE was found to decrease with an increase in the sintering time. However, this trend could not be explained by the authors.

    Canto and co-workers3 subjected PTFE powder granules to a nonstandard cyclic DSC test consisting of successive heating and cooling cycles at between 240 and 360 C with different cooling rates. The degree of crystallinity of PTFE was estimated from the ratio between the enthalpies of crystallization and subsequent melting. This strategy allowed the degree of crystallinity to be estimated as a logarithmic function of the cooling rate from a single sample. A slight nonlinearity was obtained but the authors did not address this and applied a linear fitting.

    Strabelli and co-workers4 investigated the influence of sintering variables on the microstructure and degree of crystallinity of isostatically cold-pressed PTFE parts. Sintering was carried out in an air-circulating oven at three different temperatures (360, 375, 390 oC) for times ranging from 10 to 10,000 min. The degree of crystallinity was estimated by

    density measurements and DSC using samples taken from the core of the parts. Data obtained from these two techniques showed good agreement. For these time intervals, the degree of crystallinity was found to increase during the sintering for samples heat-treated at 360 and 375 oC, while the sample sintered at 390 oC showed a maximum at 1,000 min.

    Ganguly and Lesser23 investigated the crystallization of PTFE subjected to cyclic thermal loading and isothermal treatment by DSC. Samples were cold pressed in two steps until reaching 2.15 g cm-3. Sintering was performed by DSC applying a relatively long time interval (up to 2,000 min). It was found that the melting enthalpy of the PTFE samples increased with sintering time. Moreover, the melting enthalpies of the samples subjected to cyclic thermal loading were equivalent to those of the samples subjected to an isothermal condition, for the same sintering time.

    Glaris and co-workers25, using scanning electron microscopy (SEM), observed the presence of aspherical structures, referred to as warts, on the external surface of PTFE samples subjected to heat treatment in an oven. The heat treatment was replicated in a DSC instrument and the melting enthalpy was measured. The degree of crystallinity was estimated from the ratio between the enthalpies of two subsequent melting events. The authors found that an increase in the degree of crystallinity was associated with an increase in the formation